Chinese spies have breached the personal email accounts of many top Obama Administration officials and have been reading their emails since 2010, according to a report from NBC. This has got to be bad for Hillary Clinton, whose use of personal email address as Secretary of State during that time is now under FBI investigation.

NBC is basing its report on a document obtained from the NSA in addition to anonymous sources, which suggests the NSA is aware of the intrusion but hasn't been able to stop it:

The email grab -- first codenamed "Dancing Panda" by U.S. officials, and then "Legion Amethyst" -- was detected in April 2010, according to a top secret NSA briefing from 2014. The intrusion into personal emails was still active at the time of the briefing and, according to the senior official, is still going on.
In 2011, Google disclosed that the private gmail accounts of some U.S. officials had been compromised, but the briefing shows that private email accounts from other providers were compromised as well.

NBC hasn't confirmed that Clinton is among the hacked officials, but if she is, she'll be in a distinctly vulnerable position. Clinton rigged up a home-brewed email system and used it to conduct official business. Clinton relied far more heavily on her personal email address while she was in office than other officials, which means the contents of her emails likely contain more officially compromising information.